When starting gas turbine power plants, it is often required also to start the steam turbine as soon as sufficient steam for driving the steam turbine can be provided by the waste heat of the gas turbine. For this purpose, the gas turbine and the steam turbine are coupled by means of a coupling. Particularly to avoid imbalances, ways of using a specific control of the coupling operation to couple with a target coupling angle are adopted. For this purpose, the steam turbine is accelerated in a suitable way. The frequency of the gas turbine is prescribed to the extent that it must coincide with the frequency of the power grid into which the feeding is taking place.
EP 1 911 939 A1 discloses a method for coupling an input shaft of a turbo machine to an output shaft by means of a coupling. The turbo machine is brought up to a speed that is subsynchronous to the speed of the output shaft and is kept at this holding speed before a signal for starting the coupling is set to achieve coupling with the target coupling angle. The turbo machine is generally a steam turbine and the output shaft is the shaft for driving the generator.